1. The Field of the Invention
The present invention relates to composite wall structures and, more specifically to the field of connectors used to secure together multiple layers of material within the composite wall structures.
2. The Relevant Technology
As new materials and compositions have been developed, apparently unrelated materials have been synergistically combined to form useful composite materials. One such example is seen in the area of building and construction, in which high strength structural walls have been coated and layered with highly insulative materials which generally have relatively low structural strength. The resulting composite wall structure has high strength and is highly insulative. Conventionally, the structural component of such as a wall is built first, after which the insulating layer or sheet is attached to the structural component. Thereafter a protective cover is placed over the insulating material to protect and hide it. The insulating barrier reduces the transfer of thermal energy across the composite wall structure.
Concrete is one of the least expensive and strongest materials commonly used in the construction industry. Unfortunately, concrete, which is a mixture of hydraulic cement, water, and an aggregate such as rocks, pebbles, and sand, offers relatively poor insulation compared to many other materials. For example, a slab of concrete having an 8 inch thickness has an R value of about 0.64, while a one-inch thick panel of polystyrene has an R value of about 5.0. The R value of a material is proportional to the thermal resistance of the material and is useful for comparing the insulating properties of materials used in the construction industry.
In contrast with concrete, highly insulative materials, at least those of reasonable cost, typically offer poor structural strength and integrity. While lightweight aggregates having higher insulating ability may be incorporated within concrete to increase the insulating effect thereof, the use of such aggregates in an amount that has a dramatic effect on the insulation ability of the concrete will usually result in greatly decreased strength of the resulting structure.
It has been found that positioning at least one concrete layer adjacent to at least one insulating layer provides a composite wall structure that has both good insulating capability and good structural strength. One strategy for forming these composite wall structures is to position an insulating layer between two concrete layers. This technique, however, poses the risk of allowing the two concrete layers to collapse together or to separate apart during construction or subsequent use of the building. Accordingly, it is necessary to structurally bridge or connect the two concrete layers together. This is conventionally accomplished by using metal casting form ties.
Because metal readily conducts thermal energy, metal casting form ties that are used to structurally bridge a pair of structural layers have the effect of significantly reducing the insulating properties of a composite wall. In particular, such casting form ties provide channels through which thermal energy may be conducted. This is true even though the ties may be surrounded by ample amounts of insulating material. Composite wall structures that use metal casting form ties do not prevent heat from flowing from a relatively warm inside wall to a colder outside wall during cold weather, for example, as effectively as composite walls that do not use metal casting form ties. Of course one might construct a building having no structural bridges between the inner and outer structural walls, although the result would be a building having inadequate stability for most needs.
In order to reduce thermal bridging, some have employed casting form ties having a metal portion that passes through the concrete layers and a thermally insulating portion that passes through the insulating layer, e.g., U.S. Pat. No. 4,545,163 to Asselin. Others have developed casting form ties made entirely from polymeric or other highly insulative materials. Examples of the foregoing include U.S. Pat. No. 4,829,733 to Long; U.S. Pat. No. 5,519,973 to Keith et al.; U.S. Pat. No. 5,606,832 to Keith et al.; U.S. Pat. No. 5,673,525 to Keith et al, and U.S. Pat. No. 6,138,981 to Keith et al. For purposes of disclosing insulating casting form ties used to secure a composite wall structure together, each of the foregoing patents are incorporated herein by specific reference.
A common technique for forming composite wall structures is known in the art as the “cast-in-place” method, wherein the wall is formed within vertically positioned casting forms that are erected at or near the location where the composite wall structure is to be finally positioned. In the cast-in-place method the forms and insulating layer are first positioned vertically, after which concrete or other structural material is poured into the spaces between the insulating layer and casting forms. Insulating or metal casting form ties having a length that is more than, equal to or less than the width of the composite wall structure are placed substantially orthogonally through a vertically oriented insulating layer, with the ends of the ties extending out of either surface of the insulating layer.
Composite action, which is well known by those skilled in the art, generally describes how well a multi-layered panel, or composite wall, transfers shear forces between its different layers and is typically identified as a percentage between 0% and 100%. A layered panel having a very high composite action will transfer shear forces very well and will behave like a single laminated panel. Whereas, a layered panel having a very low composite action will not transfer shear forces well and will behave more like a panel having a plurality of disconnected layers. Composite action can provide structural integrity to the wall, although too much composite action in a tall above grade wall may result in too much bowing of the wall because of differential thermal expansion when the inside and outside surfaces of the wall are subjected to different thermal environments. Accordingly, it is generally desirable to produce composite walls having moderately high to high composite action, although the specific composite action desired depends on the wall environment.